By Jonathan Power

When it comes to war, the world does move on for the better, even as dramatic events like the U.S. invasion of Venezuela seize global attention. At present, there are no full-scale interstate wars. The longest civil wars of our era—in Afghanistan and Syria—are winding down. The Thai–Cambodian conflict splutters on at low intensity. Only Sudan’s civil war appears continuously inflammable. And only one country today openly threatens the use of nuclear weapons—North Korea—yet both its arsenal and credibility remain limited.

The world is certainly not as benign as it was in the 1990s at the end of the Cold War, a period that marked the lowest point in recorded warfare. There are more conflicts today than in that brief interlude, and the Cold War itself threatens to re-combust. Nevertheless, the number of soldiers killed on battlefields continues its long-term decline. In Europe—the historic epicenter of warfare—there has been no major war between states for roughly 80 years.

War in the early seventeenth century, at the time of the great Dutch thinker Hugo Grotius, was understood in a very different way from the way it is today. Grotius taught that war was the legitimate way in........

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